1. Field of the Invention
The present invention relates to a durable safety switch, which automatically trips off upon an overload.
2. Description of the Related Art
FIG. 1 shows a regular switch, which comprises an electrically insulative housing 100, three metal contact plates 110a-c fastened to the bottom sidewall 101 of the housing 100, a lever 120 disposed at the topside of the housing 100, a neon light 130 mounted in the lever 120, the neon light 130 having a first end disposed corresponding to one lateral metal contact plate 110c and a second end, a metal frame 150 coupled to the middle metal contact plate 110b, the metal frame 150 having a contact 151 aimed at a contact 111 at one lateral metal contact plate 110a, and a metal spring 140 connected between the metal frame 150 and the second end of the neon light 130. When one end of the lever 120 is depressed, the first end of the neon light 130 is forced into contact with the corresponding lateral metal contact plate 110a, and at the same time the spring 140 is compressed to force the metal frame 150 to tile in direction reversed to the lever 120, thereby causing the contact 151 of the metal frame 150 to be forced into contact with the contact 111 at the other lateral metal contact plate 110a, and therefore the circuit is closed. When the other end of the lever 120 is depressed, the neon light 130 is disconnected from the corresponding lateral metal contact plate 110c, and the metal frame 150 is tilted in the reversed direction and disconnected from the other lateral metal contact plate 110b to open the circuit. This structure of switch is not safe in use. In case of an overload when the circuit is closed, the metal frame 150 does not automatically trip off to open the circuit, and the load may be burnt out. FIG. 2 shows another conventional safety switch. This structure of safety switch comprises an electrically insulative housing 200, three metal contact plates 220a-c fastened to the bottom side of the housing 200, a lever 210 pivoted to the housing 200 at the top side, a metal frame 230 fixedly fastened to the middle metal contact plate 220b, a bimetal element 240 having a first end fastened to the top side of the metal frame 230 and a second end, a C-shaped spring 241 having one end pivoted to the second end of the bimetal element 240 and an opposite end connected to the bottom side of the metal frame 220, a first contact 242 disposed at the bimetal element 240, and a second contact 221 disposed at one lateral metal contact plate 220c adjacent to the first contact 242. When an overload occurs, the bimetal element 240 is heated and disengages from contact with the contact 221 of the metal contact plate 220c trip to cut off the power supply. This structure of switch is safe in use. However, this structure of safety switch is not durable because the bimetal element 240 breaks quickly with repeated use.